1. Field of the Invention
The present invention relates to a multilayer ceramic substrate and a method for manufacturing the same, and more particularly, to a multilayer ceramic substrate that is provided with external conductive films on surfaces thereof and that is manufactured by a non-shrinking process and to a method for manufacturing the multilayer ceramic substrate.
2. Description of the Related Art
A technique of interest to the present invention has been disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2001-291959. Japanese Unexamined Patent Application Publication No. 2001-291959 discloses a method for manufacturing a multilayer ceramic substrate by firing a laminate including base green sheets including a low temperature sintering ceramic material and shrinkage suppressing inorganic material layers including a shrinkage suppressing inorganic material laminated thereto, which attempts to solve the following problem. The problem is that at the stage at which a shrinkage suppressing effect of the shrinkage suppressing inorganic material layers does not act on the base green sheets during a firing step, a conductive paste body defining wire conductors provided on a green laminate starts to shrink, and as a result, in an obtained multilayer ceramic substrate, cracks, bumps, and gaps may be generated. In Japanese Unexamined Patent Application Publication No. 2001-291959, in order to solve the problem, the conductive paste body is formed using a copper-based conductive paste including Cu2O, and an oxygen partial pressure in a firing atmosphere is controlled during a firing step so as to make the shrinkage behavior generated when Cu2O is reduced to Cu similar to that of the low temperature sintering ceramic material included in the base green sheets.
However, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2001-291959 produces the following problem that must also be solved. As the wire conductors in the multilayer ceramic substrate, there are external conductive films, internal conductive films, and via hole conductors. When conductive paste films defining the external conductive films are fired simultaneously with the green laminate, the green laminate does not substantially shrink in a primary surface direction but shrinks only in a lamination direction, and the conductive paste films shrink in isotropic directions including the primary surface direction. Thus, a tensile stress caused by the shrinkage of the conductive paste films is applied to the laminate at peripheral portions of the conductive paste films, and as a result, cracks may be generated in the multilayer ceramic substrate and/or the strength thereof may be degraded.
The problems described above are particularly likely to occur when the shrinkage suppressing inorganic material layer is arranged along the external surface of the green laminate. However, when the strength of the laminate itself is taken into account, the shrinkage suppressing inorganic material layer is effectively arranged along the external surface of the laminate. The reason for this is that when the shrinkage suppressing inorganic material layer is arranged along the external surface of the laminate, a state is obtained in which a compressive stress is applied to the external surface of the laminate after firing is performed. Due to this compressive stress, it has been difficult to depart from a configuration in which the shrinkage suppressing inorganic material layer is arranged along the external surface of the laminate.
In addition, in order to form the conductive paste film, the use of a copper-based conductive paste including Cu2O reduces the generation of cracks and a decrease in strength to a certain extent. However, these effects may not be sufficient. In addition, the conductive paste disclosed in Japanese Unexamined Patent Application Publication No. 2001-291959 includes a glass component. Where the external conductive film is formed using the conductive paste as described above, when the shrinkage suppressing inorganic material layer or the base green sheet located under the external conductive film includes glass, or when a glass component is deposited on the external surface from the inside of the laminate during a firing step, a flat glass-rich layer having poor bonding properties is formed at a bottom surface side of the external conductive film after the firing is performed, and as a result, the bonding strength of the external conductive film may be insufficient.